This invention relates to a photo-to-photo conversion element suitable for image pickup devices, optical writing projectors, and the like.
Image pickup devices using photo-to-photo conversion elements are free from problems encountered with conventional image pickup devices, and are therefore able to generate with ease a video signal for providing a reproduced pictorial image having high picture quality and high resolution.
An example of such an image pickup device constituted with photo-to-photo elements can be readily understood by making reference to Japanese Patent Application No. 311333/86 entitled "Image pickup device" filed on Dec. 30, 1986 by the assignee.
For a photo-to-photo conversion element having a structure capable of receiving an optical image and outputting an optical image also as its output, attention has been conventionally paid to elements, e.g., a liquid crystal optical modulator, a photoconductive Pockels cell, a spatial light element such as a microchannel light modulator, an element constituted by a photochromic material and the like as, e.g., an optical writing projector, an element for optical parallel processing of an optical computer, an element for recording a picture, etc. In addition, the assignee has also proposed an image pickup device having high resolution using photo-to-photo conversion elements.
FIG. 1 is a side cross-sectional view showing an example of the configuration of a conventional photo-to-photo element. The photo-to-photo conversion element shown in FIG. 1 has an arrangement comprising, in a stacked sequence, a glass plate 1, a transparent electrode 3, a photoconductive layer 7, a light screening or shielding layer 12, a dielectric mirror 8, an optical member (e.g., an optical modulation layer such as lithium niobate monocrystal or a nematic liquid crystal layer) 9 for changing the state of light in dependence upon an applied electric field, a transparent electrode 4, and a glass plate 2. This photo-to-photo conversion element is such that a write light WL, a read light RL, and an erase light EL are irradiated from the glass plate 1 side, the glass plate 2 side, and the glass plate 1 side, respectively.
In the photo-to-photo conversion element shown in FIG. 1, a circuit comprising a power supply 10 and a changeover switch SW is connected between terminals 5 and 6. By a switching control signal delivered to an input terminal 11 for the switching control signal in the changeover switch SW, the movable contact of the changeover switch SW is switched to the fixed contact WR side. Under this condition, a voltage of the power supply 10 is applied across the transparent electrodes 3 and 4, to therefore apply an electric field across both terminals of the optical member 9. Furthermore, when the write light WL is caused to be incident from the glass plate 1 side in the photo-to-photo conversion element to transmit the incident write light WL through the glass plate 1 and the transparent electrode 3 to reach the photoconductive layer 7, because the electric resistance value of the photoconductive layer 7 changes in correspondence to an optical image of the incident light which has reached the photoconductive layer 7, a charge image, corresponding to the optical image of the incident light which has reached the photoconductive layer 7, is produced at the boundary surface between the photoconductive layer 7 and the light screening layer 12.
Furthermore, under the condition where the movable contact of the changeover switch SW is switched to the fixed contact WR side as previously described, an electric field having a strength distribution corresponding to the charge image, having been produced by the write light as described above at the boundary surface between the photoconductive layer 7 and the light screening layer 12, is applied to the optical modulation layer 9 such as a lithium niobate monocrystal (or nematic liquid crystal layer) 9 provided so as to have a serial relationship with respect to the above-mentioned photoconductive layer 7 along with the light screening layer 12 and the dielectric mirror 8, etc. between transparent electrodes 1 and 2 across which a voltage of the power supply 10 is applied through terminals 5 and 6. Thus, the read light RL incident from the glass plate 2 side changes to a reflected light including pictorial image information corresponding to the strength of an electric field applied to the optical modulation layer 9 by the electro-optic effect of the optical modulation layer 9, and is thus emitted from the glass plate 2 side.
The light not reflected by the dielectric mirror 8 of the read light incident to the glass plate 2 side, as stated above, goes on through a path including the transparent electrode 4, the optical modulation layer 9, the dielectric mirror 8 and the light screening layer 12, and is screened by the light screening layer 12 so that it does not go further toward the photoconductive layer 7 side. For this reason, even if the read light RL is incident or projected to the glass plate 2 side, there is no possibility that the electric resistance value of the photoconductive layer 7 changes thereby. Thus, even read light RL incident to the photo-to-photo conversion element gives no possibility of a change in the charge image produced in correspondence with the optical image by the incident light at the boundary surface between the photoconductive layer 7 and the light screening layer 12.
On the other hand, in the above-mentioned photo-to-photo conversion element shown in FIG. 1, the information written thereinto by the write light WL will be erased as follows. First, a switching control signal is delivered to the input terminal 11 for the switching control signal in the changeover switch SW to switch the movable contact of the chageover switch SW to the fixed contact E side, to therefore allow the terminals 5 and 6 to have the same potential so that no electric field is produced across the transparent electrodes 3 and 4. Then, by allowing an erase light EL having a uniform strength distribution to be incident from the glass plate 1 side which is the incident side of the write light WL, the erase light EL is given to the photoconductive layer 7 through the glass plate 1 and the transparent electrode 3, thus placing the photoconductive layer 7 in the state where its electric resistance value is lowered to erase a charge image produced at the boundary surface between the photoconductive layer 7 and the light screening layer 12.
The reason why the side on which the erase light is incident for erasing the information having been already written into the conventional photo-to-photo conversion element shown in FIG. 1 stated above, is the same as side on which the write light WL in incident is as follows. Since there is the light screening layer 12 between the side of which the read light RL and the photoconductive layer 7, even if the erase light is caused to be some from the incident side on which the read light RL is incident, that erase light is stopped by the light screening layer 12, and therefore failing to reach the photoconductive layer 7. Accordingly, even if the erase light is caused to be same from the incident side on which the read light RL is incident, a charge image occurring at the boundary surface between the photoconductive layer 7 and the light screening layer 12 cannot be erased.
This is a serious problem when the photo-to-photo element is used, e.g., in an image pickup device of a structure such that it is required to provide an image pickup optical system on the side on which the write light WL is incident, or in a device of a structure such that it is difficult to provide the incident unit for the erase light on the side on which the write light WL is incident.
For a photo-to-photo conversion element which can solve the above-mentioned problem, the assignee has already proposed a photo-to-photo conversion element constructed as shown in FIG. 2, i.e., a photo-to-photo conversion element comprising, in a stacked sequence, a glass plate 1, a transparent electrode 3, a photoconductive layer 7, an optical member 8R having wavelength selectivity permitting a light in the wavelength region of the read light to be reflected and permitting light in the wavelength region of the erase light to be transmitted, an optical member 9 for changing the state of light in dependence upon an applied electric field, a transparent electrode 4, and a glass plate 2.
For such an optical member 8R, e.g., a member constituted by a dichroic filter comprised of a multilayer layer consisting of a thin film of SiO.sub.2 and a thin plate of TiO.sub.2 may be used.
Moreover, for the optical member 9, e.g., an electro-optic effect crystal such as a lithium niobate monocrystal, or an optical member constituted by a nematic liquid crystal layer may be used. In FIG. 2, WL, RL and EL denote a write light, a read light and an erase light, respectively.
FIGS. 3A to 3D are characteristic curves showing the wavelength selection characteristic of the optical member 8R having a wavelength selectivity permitting a light in the wavelength region or band of the read light to be reflected and permitting a light in the wavelength region of the erase light to be transmitted, respectively. FIG. 3 shows that the optical member 8R having a characteristic shown in FIG. 3A is constituted as an optical low-pass filter, the optical member 8R having a characteristic shown in FIG. 3B is constituted as an optical high-pass filter, the optical member 8R having a characteristic shown in FIG. 3C is constituted as an optical band-pass filter, and the optical member 8R having a characteristic shown in FIG. 3D is constituted as an optical band-rejection filter.
Namely, in the photo-to-photo conversion element previously proposed which is shown in FIG. 2, for the optical member 8R used as a part of the components thereof, i.e., the optical member 8R permitting light in the wavelength region of the read light to be reflected and permitting light in the wavelength region of the erase right to be transmitted, optical members 8R having such wavelength selectivities as shown in FIGS. 3A to 3D may be used.
In the photo-to-photo conversion element previously proposed, which is provided with the optical member 8R capable of arbitrarily having any one of the wavelength selectivities as shown in FIGS. 3A to 3D, light in a wavelength region where the transmission factor of light is low in the optical member 8R, is used as the read light to be incident to that photo-to-photo conversion element, and light in a wavelength region where the transmission factor of light is high in the optical member 8R, is used as the erase light to be incident to the photo-to-photo conversion element. Thus, the photo-to-photo conversion element previously proposed permits an erase light to be same from the incident side on which the read light is incident.
In the case of writing optical information into the photo-to-photo conversion element previously proposed which has the arrangement shown in FIG. 2, a circuit composed of power supply 10 and changeover switch SW is connected between terminals 5 and 6 of the photo-to-photo conversion element to allow the movable contact of the changeover switch SW to be switched to the fixed contact WR side by a switching control signal delivered to the input terminal 11 for the changeover control signal in the changeover switch SW. Under this condition, a voltage of the power supply 10 is applied across the transparent electrodes 3 and 4, to therefore apply an electric field across both terminals of the photoconductive layer 7. Furthermore, when the write light WL is incident from the glass plate 1 side in the photo-to-photo conversion element, writing of the optical information into the photo-to-photo conversion element will be conducted as follows.
Namely, when the write light WL incident to the photo-to-photo conversion element is transmitted through the glass plate 1 and the transparent electrode 3 to reach the photoconductive layer 7, because the electric resistance value of the photoconductive layer 7 changes in correspondence to an optical image of the incident light which has reached the photoconductive layer 7, a charge image corresponding to the optical image of the incident light which has reached the photoconductive layer 7 is produced at the boundary surface between the photoconductive layer 7 and the optical member 8R.
In order to reproduce, from the photo-to-photo conversion element, the optical information which has been written in the form of a charge image in correspondence with the optical image of the incident light in a manner as stated above, there may be employed a method to switch the movable contact of the changeover switch SW to the fixed contact WR side to apply a voltage of the power supply 10 across the transparent electrodes 3 and 4 through the terminals 5 and 6 to emit or project, from the glass plate 2 side, the read light RL having a fixed optical intensity from a light source (not shown).
Namely, since a charge image corresponding to the optical image of the incident light which has reached the photoconductive layer 7 is produced at the boundary surface between the photoconductive layer 7 and the optical member 8R in the photo-to-photo conversion element into which writing of the optical information by the incident light has been conducted as previously described, an electric field having a strength distribution corresponding to the optical image of the incident light is applied to the optical member (e.g., lithium niobate monocrystal) 9 provided so as to have a serial relationship with respect to the photoconductive layer 7 along with the optical member 8R.
Since the refractive index of the lithium niobate monocrystal 9 changes in accordance with an electric field by the electro-optic effect, the refractive index of the lithium niobate monocrystal 9 provided so as to have a serial relationship with respect to the photoconductive layer 7 along with the optical member 8R under condition where an electric field having a strength distribution corresponding to the optical image by the incident light is applied to the lithium niobate monocrystal 9, changes in accordance with a charge image produced in correspondence with the optical image of the incident light, which has reached the photoconductive layer 7, at the boundary surface between the photoconductive layer 7 and the optical member 8R in the photo-to-photo conversion element by writing of the optical information of the incident light a previously described.
Thus, when the read light RL is projected to the glass plate 2 side, the read light RL which has been projected to the glass plate 2 side goes on through the transparent electrode 4, the lithium niobate monocrystal 9 and the optical member 8R.
The above-mentioned read light RL is reflected by the optical member 8R and is then returned to the glass plate 2 side as reflected light. Since the refractive index of the lithium niobate monocrystal 9 changes in accordance with an electric field by the electro-optic effect, the reflected light of the read light RL includes pictorial image information corresponding to the strength distribution of an electric field applied to the lithium niobate monocrystal 9 by the electro-optic effect of the lithium niobate monocrystal 9, thus allowing a reproduced optical image corresponding to the optical image of the incident light to be produced on the side of the glass plate 2.
In the above-mentioned reproducing operation, the read light which has been projected from the glass plate 2 side, goes on towards the photoconductive layer 7 via the transparent electrode 4, the lithium niobate monocrystal 9 and the optical member 8R as previously described. Since the above-mentioned read light is reflected by the optical member 8R before it reaches the photoconductive layer 7, it traces an optical path including the lithium niobate monocrystal 9, the transparent electrode 4 and the glass plate 2. For this reason, there is no possibility that the read light RL reaches the photoconductive layer 7 to exert an adverse influence on the charge image of the incident light written thereinto.
As just described above, in accordance with the photo-to-photo conversion element previously proposed, write operation is carried out by allowing the write light WL to be incident from the glass plate 1 side, and the reproduction operation is carried out by allowing the read light RL to be incident to the glass plate 2 side. The method of erasing information which has been written into the previously proposed photo-to-photo conversion element shown in FIG. 2 will now be described.
In the case of erasing information written into the previously proposed photo-to-photo conversion element shown in FIG. 2, there is employed a method to switch the movable contact of the changeover switch SW to the fixed contact E side by a switching control signal delivered to the input terminal 11 for the switching control signal in the changeover switch SW connected between the terminals 5 and 6 of the photo-to-photo conversion element, to electrically short-circuit between the transparent electrodes 3 and 4 so that the transparent electrodes 3 and 4 have the same potential to cause an electric field not to be applied across both terminals of the photoconductive layer 7, to thereafter allow the erase light EL to be incident from the glass plate 2 side in the photo-to-photo conversion element.
As described above, the erase ligth EL which has been incident to the glass plate 2 side of the photo-to-photo conversion element reaches the photoconductive layer 7 via a path including the glass plate 2, the transparent electrode 4, the lithium niobate monocrystal 9, the optical member 8R, and the photoconductive layer 7 to lower the electric resistance value of the photoconductive layer 7, to thereby erase the charge image formed at the boundary surface between the photoconductive layer 7 and the optical member 8R.
As understood from the foregoing description, the previously proposed photo-to-photo conversion element shown in FIG. 2 has an arrangement such that the charge image formed at the boundary surface between the photoconductive layer 7 and the optical member 8R at the time of the writing operation is erased by the erase light same from the incident side on which the read light is incident to photo-to-photo conversion element. Thus, the photo-to-photo conversion element can be easily applied to an image pickup device of a structure such that it is required to provide the image pickup optical system on the side on which the write light WL is incident, and a device of a structure such that it is difficult to provide the incident unit for the erase light on the same side on which the write light WL is incident. Thus, this previously proposed photo-to-photo element can satisfactorily solve the problems which afflict the conventional photo-to-photo conversion element previously described with reference to FIG. 1.
Since light incident from the glass plate 1 side generally includes light in a wavelength region broader than that of visible light, it includes light in a wavelength region of the erase light EL incident from the glass plate 2 side at the time of erasing.
In the case where light in a wavelength region determined so that it is used as the erase light is included in the light incident from the glass plate 1 side into the photo-to-photo conversion element as described above, that light is transmitted through the optical member 8R having a wavelength selectivity permitting a light in the wavelength region of the read light to be reflected and permitting a light in the wavelength region of the erase light to be transmitted, and is then emitted from the photo-to-photo conversion element via the lithium niobate monocrystal 9, the transparent electrode 4, and the glass plate 2.
Thus, in the case where a light in a wavelength region determined so that it is used as the erase light EL is included in a light incident from the glass plate 1 side into the photo-to-photo conversion element under condition where the read or reproduce light RL is incident to the glass plate 2 side for allowing the photo-to-photo conversion element to carry out the reproducing operation, that light is transmitted through the optical member 8R and is emitted from the photo-to-photo conversion element via the lithium niobate monocrystal 9, the transparent electrode 4 and the glass plate 2. For this reason, the reproduced optical information from the photo-to-photo conversion element includes optical information based on light in the wavelength region of the erase light EL included in the light incident from the glass plate 1 side in addition to the original reproduced optical information obtained as a result of the fact that the read or reproduce light RL incident to the glass plate 2 side for allowing the photo-to-photo conversion element to carry out the reproducing operation is emitted from the photo-to-photo conversion element via the glass plate 2, the lithium niobate monocrystal 9, the optical member 8R, the lithium niobate monocrystal 9, the transparent electrode 4, and the glass plate 2, resulting in the problem that correct reproduction operation is not carried out. A further problem is as follows. Light having a wavelength longer than that of visible light is ordinarily used. However, the fact that light having a wavelength longer than that of visible light is included in the reproduced information is dangerous to the human eye. Thus, it has been required to take countermeasures with respect to this.